CsHLS1-CsSCL28 module regulates compact plant architecture in cucumber.

Increased planting densities boost crop yields. A compact plant architecture facilitates dense planting. However, the mechanisms regulating compact plant architecture in cucurbits remain unclear. In this study, we identified a cucumber (Cucumis sativus) compact plant architecture (cpa1) mutant from an ethyl methane sulfonate (EMS)-mutagenized library that exhibited distinctive phenotypic traits, including reduced leaf petiole angle and leaf size. The candidate mutation causes a premature stop codon in CsaV3_1G036420, which shares similarity to Arabidopsis HOOKLESS 1 (HLS1) encoding putative histone N-acetyltransferase (HAT) protein and was named CsHLS1. Consistent with the mutant phenotype, CsHLS1 was predominantly expressed in leaf petiole bases and leaves. Constitutive overexpressing CsHLS1 in cpa1 restored the wild-type plant architecture. Knockout of CsHLS1 resulted in reduces leaf petiole angle and leaf size and as well as decreased acetylation levels. Furthermore, CsHLS1 directly interacted with CsSCL28 and negatively regulated compact plant architecture in cucumber. Importantly, CsHLS1 knockout increased the photosynthesis rate and leaf nitrogen in cucumbers, thereby maintaining cucumber yield at normal density. Overall, our research provides valuable genetic breeding resource and gene target for creating a compact plant architecture for dense cucumber planting.

An SMU Splicing Factor Complex Within Nuclear Speckles Contributes to Magnesium Homeostasis in Arabidopsis.

Mg2+ is among the most abundant divalent cations in living cells. In plants, investigations on magnesium (Mg) homeostasis are restricted to the functional characterization of Mg2+ transporters. Here, we demonstrate that the splicing factors SUPPRESSORS OF MEC-8 AND UNC-52 1 (SMU1) and SMU2 mediate Mg homeostasis in Arabidopsis (Arabidopsis thaliana). A low-Mg sensitive Arabidopsis mutant was isolated, and the causal gene was identified as SMU1 Disruption of SMU2, a protein that can form a complex with SMU1, resulted in a similar low-Mg sensitive phenotype. In both mutants, an Mg2+ transporter gene, Mitochondrial RNA Splicing 2 (MRS2-7), showed altered splicing patterns. Genetic evidence indicated that MRS2-7 functions in the same pathway as SMU1 and SMU2 for low-Mg adaptation. In contrast with previous results showing that the SMU1-SMU2 complex is the active form in RNA splicing, MRS2-7 splicing was promoted in the smu2 mutant overexpressing SMU1, indicating that complex formation is not a prerequisite for the splicing. We found here that formation of the SMU1-SMU2 complex is an essential step for their compartmentation in the nuclear speckles, a type of nuclear body enriched with proteins that participate in various aspects of RNA metabolism. Taken together, our study reveals the involvement of the SMU splicing factors in plant Mg homeostasis and provides evidence that complex formation is required for their intranuclear compartmentation.

Callose Synthesis Suppresses Cell Death Induced by Low-Calcium Conditions in Leaves.

Despite the importance of preventing calcium (Ca) deficiency disorders in agriculture, knowledge of the molecular mechanisms underlying plant adaptations to low-Ca conditions is limited. In this study, we provide evidence for a crucial involvement of callose synthesis in the survival of Arabidopsis (Arabidopsis thaliana) under low-Ca conditions. A mutant sensitive to low-Ca conditions, low calcium sensitive3 (lcs3), exhibited high levels of cell death in emerging leaves and had defects in its expanding true leaves under low-Ca conditions. Further analyses showed that the causal mutation was located in a putative ß-1,3-glucan (callose) synthase gene, GLUCAN SYNTHASE-LIKE10 (GSL10). Yeast complementation assay results showed that GSL10 encodes a functional callose synthase. Ectopic callose significantly accumulated in wild-type plants under low-Ca conditions, but at a low level in lcs3 The low-Ca sensitivity of lcs3 was phenocopied by the application of callose synthase inhibitors in wild-type plants, which resulted in leaf expansion failure, cell death, and reduced ectopic callose levels under low-Ca conditions. Transcriptome analyses showed that the expression of genes related to cell wall and defense responses was altered in both wild-type plants under low-Ca conditions and in lcs3 under normal-Ca conditions, suggesting that GSL10 is required for the alleviation of both cell wall damage and defense responses caused by low Ca levels. These results suggest that callose synthesis is essential for the prevention of cell death under low-Ca conditions and plays a key role in plants' survival strategies under low-Ca conditions.

Induction of S-nitrosoglutathione reductase protects root growth from ammonium toxicity by regulating potassium homeostasis in Arabidopsis and rice.

Ammonium (NH4+) is toxic to root growth in most plants already at moderate levels of supply, but mechanisms of root growth tolerance to NH4+ remain poorly understood. Here, we report that high levels of NH4+ induce nitric oxide (NO) accumulation, while inhibiting potassium (K+) acquisition via SNO1 (sensitive to nitric oxide 1)/SOS4 (salt overly sensitive 4), leading to the arrest of primary root growth. High levels of NH4+ also stimulated the accumulation of GSNOR (S-nitrosoglutathione reductase) in roots. GSNOR overexpression improved root tolerance to NH4+. Loss of GSNOR further induced NO accumulation, increased SNO1/SOS4 activity, and reduced K+ levels in root tissue, enhancing root growth sensitivity to NH4+. Moreover, the GSNOR-like gene, OsGSNOR, is also required for NH4+ tolerance in rice. Immunoblotting showed that the NH4+-induced GSNOR protein accumulation was abolished in the VTC1- (vitamin C1) defective mutant vtc1-1, which is hypersensititive to NH4+ toxicity. GSNOR overexpression enhanced vtc1-1 root tolerance to NH4+. Our findings suggest that induction of GSNOR increases NH4+ tolerance in Arabidopsis roots by counteracting NO-mediated suppression of tissue K+, which depends on VTC1 function.

Melatonin: A master regulator of plant development and stress responses.

Melatonin is a pleiotropic molecule with multiple functions in plants. Since the discovery of melatonin in plants, numerous studies have provided insight into the biosynthesis, catabolism, and physiological and biochemical functions of this important molecule. Here, we describe the biosynthesis of melatonin from tryptophan, as well as its various degradation pathways in plants. The identification of a putative melatonin receptor in plants has led to the hypothesis that melatonin is a hormone involved in regulating plant growth, aerial organ development, root morphology, and the floral transition. The universal antioxidant activity of melatonin and its role in preserving chlorophyll might explain its anti-senescence capacity in aging leaves. An impressive amount of research has focused on the role of melatonin in modulating postharvest fruit ripening by regulating the expression of ethylene-related genes. Recent evidence also indicated that melatonin functions in the plant's response to biotic stress, cooperating with other phytohormones and well-known molecules such as reactive oxygen species and nitric oxide. Finally, great progress has been made towards understanding how melatonin alleviates the effects of various abiotic stresses, including salt, drought, extreme temperature, and heavy metal stress. Given its diverse roles, we propose that melatonin is a master regulator in plants.

Three forms of cocaine- and amphetamine-regulated transcript may be involved in food intake regulation in gibel carp (Carassius auratus gibelio).

In fish, as in mammals, several studies have demonstrated that the cocaine- and amphetamine-regulated transcript (CART) plays an important role in feeding. However, thus far, the function of CART in gibel carp (Carassius auratus gibelio) feeding regulation has not been reported. In our study, we first identified three forms of CART peptide precursors from gibel carp brain and named these CART-1, CART-2, and CART-3. The full-length cDNA sequences of CART-1, CART-2, and CART-3 were 616 bp, 705 bp, and 760 bp, respectively, encoding peptides of 118, 120, and 104 amino acid residues. We detected mRNA expression of CART-1, CART-2, and CART-3 in a wide range of peripheral and central tissues, with the highest expression detected in the brain. After a meal, mRNA expression of CART-1, CART-2, and CART-3 was significantly elevated, suggesting that CART-1, CART-2, and CART-3 may act as postprandial satiety signals. Moreover, mRNA expression of all three CART-1, CART-2, and CART-3 was significantly reduced during fasting and significantly elevated with refeeding. Our findings indicate that CART-1, CART-2, and CART-3 might function as a satiety factor in the gibel carp.

Auxin Resistant1 and PIN-FORMED2 Protect Lateral Root Formation in Arabidopsis under Iron Stress.

A stunted root system is a significant symptom of iron (Fe) toxicity, yet little is known about the effects of excess Fe on lateral root (LR) development. In this work, we show that excess Fe has different effects on LR development in different portions of the Arabidopsis (Arabidopsis thaliana) root system and that inhibitory effects on the LR initiation are only seen in roots newly formed during excess Fe exposure. We show that root tip contact with Fe is both necessary and sufficient for LR inhibition and that the auxin, but not abscisic acid, pathway is engaged centrally in the initial stages of excess Fe exposure. Furthermore, Fe stress significantly reduced PIN-FORMED2 (PIN2)-green fluorescent protein (GFP) expression in root tips, and pin2-1 mutants exhibited significantly fewer LR initiation events under excess Fe than the wild type. Exogenous application of both Fe and glutathione together increased PIN2-GFP expression and the number of LR initiation events compared with Fe treatment alone. The ethylene inhibitor aminoethoxyvinyl-glycine intensified Fe-dependent inhibition of LR formation in the wild type, and this inhibition was significantly reduced in the ethylene overproduction mutant ethylene overproducer1-1. We show that Auxin Resistant1 (AUX1) is a critical component in the mediation of endogenous ethylene effects on LR formation under excess Fe stress. Our findings demonstrate the relationship between excess Fe-dependent PIN2 expression and LR formation and the potential role of AUX1 in ethylene-mediated LR tolerance and suggest that AUX1 and PIN2 protect LR formation in Arabidopsis during the early stages of Fe stress.

Physiological and molecular mechanisms of plant-root responses to iron toxicity.

The chemical form and physiological activity of iron (Fe) in soil are dependent on soil pH and redox potential (Eh), and Fe levels in soils are frequently elevated to the point of causing Fe toxicity in plants, with inhibition of normal physiological activities and of growth and development. In this review, we describe how iron toxicity triggers important physiological changes, including nitric-oxide (NO)-mediated potassium (K+) efflux at the tips of roots and accumulation of reactive oxygen species (ROS) and reactive nitrogen (RNS) in roots, resulting in physiological stress. We focus on the root system, as the first point of contact with Fe in soil, and describe the key processes engaged in Fe transport, distribution, binding, and other mechanisms that are drawn upon to defend against high-Fe stress. We describe the root-system regulation of key physiological processes and of morphological development through signaling substances such as ethylene, auxin, reactive oxygen species, and nitric oxide, and discuss gene-expression responses under high Fe. We especially focus on studies on the physiological and molecular mechanisms in rice and Arabidopsis under high Fe, hoping to provide a valuable theoretical basis for improving the ability of crop roots to adapt to soil Fe toxicity.

Novel lignin-based extracellular barrier in glandular trichome.

Successful biochemical reactions in organisms necessitate compartmentalization of the requisite components. Glandular trichomes (GTs) act as compartments for the synthesis and storage of specialized compounds. These compounds not only are crucial for the survival of plants under biotic and abiotic stresses but also have medical and commercial value for humans. However, the mechanisms underlying compartmentalization remain unclear. Here we identified a novel structure that is indispensable for the establishment of compartments in cucumber GTs. Silica, a specialized compound, is deposited on the GTs and is visible on the surface of the fruit as a white powder, known as bloom. This deposition provides resistance against pathogens and prevents water loss from the fruits1. Using the cucumber bloomless mutant2, we discovered that a lignin-based cell wall structure in GTs, named 'neck strip', achieves compartmentalization by acting as an extracellular barrier crucial for the silica polymerization. This structure is present in the GTs of diverse plant species. Our findings will enhance the understanding of the biosynthesis of unique compounds in trichomes and provide a basis for improving the production of compounds beneficial to humans.

GSA-1/ARG1 protects root gravitropism in Arabidopsis under ammonium stress.

Gravitropism plays a critical role in plant growth and development, plant stability and acclimation to changes in water and nutrient availability. Ammonium (NH4(+)) is well known to have profound effects on root growth, but its impacts on gravitropism are poorly understood. To determine which genes are essential for the maintenance of root gravitropism under NH4(+) stress, we isolated and identified an NH4 (+)-sensitive mutant, gsa-1 (gravitropism sensitive to ammonium-1), in Arabidopsis thaliana, using an agar plate root reorientation assay. We found that, under NH4(+) stress, gsa-1 displayed increased loss of root gravitropism. Gene cloning and sequencing revealed that gsa-1 contains a G to C transversion mutation at the highly conserved 5'-GT splice position of intron 10 of ARG1 (ALTERED RESPONSE TO GRAVITY1), known to participate in the transduction of the root gravity signal. Genetic complement tests established the locus of GSA-1/ARG1 and its role in resistance to NH4 (+) inhibition on root gravitropism. GSA-1/ARG1 is required for normal AUX1 expression and basipetal auxin transport in root apices. In addition, PIN-FORMED2 (PIN2) is proposed as a target in the reduction of root gravitropism under NH4(+) stress, a response which can be antagonized by the GSA-1/ARG1-dependent pathway. These results suggest that GSA-1/ARG1 protects root gravitropism in Arabidopsis thaliana under ammonium stress.

Arabidopsis plastid AMOS1/EGY1 integrates abscisic acid signaling to regulate global gene expression response to ammonium stress.

Ammonium (NH(4)(+)) is a ubiquitous intermediate of nitrogen metabolism but is notorious for its toxic effects on most organisms. Extensive studies of the underlying mechanisms of NH(4)(+) toxicity have been reported in plants, but it is poorly understood how plants acclimate to high levels of NH(4)(+). Here, we identified an Arabidopsis (Arabidopsis thaliana) mutant, ammonium overly sensitive1 (amos1), that displays severe chlorosis under NH(4)(+) stress. Map-based cloning shows amos1 to carry a mutation in EGY1 (for ethylene-dependent, gravitropism-deficient, and yellow-green-like protein1), which encodes a plastid metalloprotease. Transcriptomic analysis reveals that among the genes activated in response to NH(4)(+), 90% are regulated dependent on AMOS1/EGY1. Furthermore, 63% of AMOS1/EGY1-dependent NH(4)(+)-activated genes contain an ACGTG motif in their promoter region, a core motif of abscisic acid (ABA)-responsive elements. Consistent with this, our physiological, pharmacological, transcriptomic, and genetic data show that ABA signaling is a critical, but not the sole, downstream component of the AMOS1/EGY1-dependent pathway that regulates the expression of NH(4)(+)-responsive genes and maintains chloroplast functionality under NH(4)(+) stress. Importantly, abi4 mutants defective in ABA-dependent and retrograde signaling, but not ABA-deficient mutants, mimic leaf NH(4)(+) hypersensitivity of amos1. In summary, our findings suggest that an NH(4)(+)-responsive plastid retrograde pathway, which depends on AMOS1/EGY1 function and integrates with ABA signaling, is required for the regulation of expression of NH(4)(+)-responsive genes that maintain chloroplast integrity in the presence of high NH(4)(+) levels.

Ammonium-induced shoot ethylene production is associated with the inhibition of lateral root formation in Arabidopsis.

Foliar NH4(+) exposure is linked to inhibition of lateral root (LR) formation. Here, the role of shoot ethylene in NH4(+)-induced inhibition of LR formation in Arabidopsis was investigated using wild-type and mutant lines that show either blocked ethylene signalling (etr1) or enhanced ethylene synthesis (eto1, xbat32). NH4(+) exposure of wild-type Arabidopsis led to pronounced inhibition of LR production chiefly in the distal root, and triggered ethylene evolution and enhanced activity of the ethylene reporter EBS:GUS in the shoot. It is shown that shoot contact with NH4(+) is necessary to stimulate shoot ethylene evolution. The ethylene antagonists Ag(+) and aminoethoxyvinylglycine (AVG) mitigated LR inhibition under NH4(+) treatment. The decrease in LR production was significantly greater for eto1-1 and xbat32 and significantly less for etr1-3. Enhanced shoot ethylene synthesis/signalling blocked recovery of LR production when auxin was applied in the presence of NH4(+) and negatively impacted shoot AUX1 expression. The findings highlight the important role of shoot ethylene evolution in NH4(+)-mediated inhibition of LR formation.

A lignin-derived material improves plant nutrient bioavailability and growth through its metal chelating capacity.

The lignocellulosic biorefinery industry can be an important contributor to achieving global carbon net zero goals. However, low valorization of the waste lignin severely limits the sustainability of biorefineries. Using a hydrothermal reaction, we have converted sulfuric acid lignin (SAL) into a water-soluble hydrothermal SAL (HSAL). Here, we show the improvement of HSAL on plant nutrient bioavailability and growth through its metal chelating capacity. We characterize HSAL's high ratio of phenolic hydroxyl groups to methoxy groups and its capacity to chelate metal ions. Application of HSAL significantly promotes root length and plant growth of both monocot and dicot plant species due to improving nutrient bioavailability. The HSAL-mediated increase in iron bioavailability is comparable to the well-known metal chelator ethylenediaminetetraacetic acid. Therefore, HSAL promises to be a sustainable nutrient chelator to provide an attractive avenue for sustainable utilization of the waste lignin from the biorefinery industry.

Isolation and characterization of a novel ammonium overly sensitive mutant, amos2, in Arabidopsis thaliana.

Ammonium (NH(4)(+)) toxicity is a significant agricultural problem globally, compromising crop growth and productivity in many areas. However, the molecular mechanisms of NH(4)(+) toxicity are still poorly understood, in part due to a lack of valuable genetic resources. Here, a novel Arabidopsis mutant, amos2 (ammonium overly sensitive 2), displaying hypersensitivity to NH(4) (+) in both shoots and roots, was isolated. The mutant exhibits the hallmarks of NH(4)(+) toxicity at significantly elevated levels: severely suppressed shoot biomass, increased leaf chlorosis, and inhibition of lateral root formation. Amos2 hypersensitivity is associated with excessive NH(4)(+) accumulation in shoots and a reduction in tissue potassium (K(+)), calcium (Ca(2+)), and magnesium (Mg(2+)). We show that the lesion is specific to the NH(4)(+) ion, is independent of NH(4)(+) metabolism, and can be partially rescued by elevated external K(+). The amos2 lesion was mapped to a 16-cM interval on top of chromosome 1, where no similar mutation has been previously mapped. Our study identifies a novel locus controlling cation homeostasis under NH(4)(+) stress and provides a tool for the future identification of critical genes involved in the development of NH(4)(+) toxicity.

Ammonium-induced loss of root gravitropism is related to auxin distribution and TRH1 function, and is uncoupled from the inhibition of root elongation in Arabidopsis.

Root gravitropism is affected by many environmental stresses, including salinity, drought, and nutrient deficiency. One significant environmental stress, excess ammonium (NH(4)(+)), is well documented to inhibit root elongation and lateral root formation, yet little is known about its effects on the direction of root growth. We show here that inhibition of root elongation upon elevation of external NH(4)(+) is accompanied by a loss in root gravitropism (agravitropism) in Arabidopsis. Addition of potassium (K(+)) to the treatment medium partially rescued the inhibition of root elongation by high NH(4)(+) but did not improve gravitropic root curvature. Expression analysis of the auxin-responsive reporter gene DR5::GUS revealed that NH(4)(+) treatment delayed the development of gravity-induced auxin gradients across the root cap but extended their duration once initiated. Moreover, the ß-glucuronidase (GUS) signal intensity in root tip cells was significantly reduced under high NH(4)(+) treatment over time. The potassium carrier mutant trh1 displayed different patterns of root gravitropism and DR5::GUS signal intensity in root apex cells compared with the wild type in response to NH(4)(+). Together, the results demonstrate that the effects of NH(4)(+) on root gravitropism are related to delayed lateral auxin redistribution and the TRH1 pathway, and are largely independent of inhibitory effects on root elongation.

CASP microdomain formation requires cross cell wall stabilization of domains and non-cell autonomous action of LOTR1.

Efficient uptake of nutrients in both animal and plant cells requires tissue-spanning diffusion barriers separating inner tissues from the outer lumen/soil. However, we poorly understand how such contiguous three-dimensional superstructures are formed in plants. Here, we show that correct establishment of the plant Casparian Strip (CS) network relies on local neighbor communication. We show that positioning of Casparian Strip membrane domains (CSDs) is tightly coordinated between neighbors in wild-type and that restriction of domain formation involves the putative extracellular protease LOTR1. Impaired domain restriction in lotr1 leads to fully functional CSDs at ectopic positions, forming 'half strips'. LOTR1 action in the endodermis requires its expression in the stele. LOTR1 endodermal expression cannot complement, while cortex expression causes a dominant-negative phenotype. Our findings establish LOTR1 as a crucial player in CSD positioning acting in a directional, non-cell-autonomous manner to restrict and coordinate CS positioning.

Shoot-supplied ammonium targets the root auxin influx carrier AUX1 and inhibits lateral root emergence in Arabidopsis.

Deposition of ammonium (NH4+) from the atmosphere is a substantial environmental problem. While toxicity resulting from root exposure to NH4+ is well studied, little is known about how shoot-supplied ammonium (SSA) affects root growth. In this study, we show that SSA significantly affects lateral root (LR) development. We show that SSA inhibits lateral root primordium (LRP) emergence, but not LRP initiation, resulting in significantly impaired LR number. We show that the inhibition is independent of abscisic acid (ABA) signalling and sucrose uptake in shoots but relates to the auxin response in roots. Expression analyses of an auxin-responsive reporter, DR5:GUS, and direct assays of auxin transport demonstrated that SSA inhibits root acropetal (rootward) auxin transport while not affecting basipetal (shootward) transport or auxin sensitivity of root cells. Mutant analyses indicated that the auxin influx carrier AUX1, but not the auxin efflux carriers PIN-FORMED (PIN)1 or PIN2, is required for this inhibition of LRP emergence and the observed auxin response. We found that AUX1 expression was modulated by SSA in vascular tissues rather than LR cap cells in roots. Taken together, our results suggest that SSA inhibits LRP emergence in Arabidopsis by interfering with AUX1-dependent auxin transport from shoot to root.

The Emerging Role of GSNOR in Oxidative Stress Regulation.

Oxidative stress is a common event in aerobic organisms and a fundamental and unavoidable cost of the aerobic lifestyle. Reactive oxygen and nitrogen species (ROS/RNS) and iron (Fe) are the most common agents that trigger oxidative stress. A conserved enzyme in the S-nitrosoglutathione (GSNO) metabolism, GSNO reductase (GSNOR), modulates a multitude of abiotic and biotic stress responses. In this review, we focus on the emerging role of GSNOR as a master regulator in oxidative stress through its regulation of the interaction of ROS, RNS, and Fe, and highlight recent discoveries in post-translational modifications of GSNOR and functional variations of natural GSNOR variants during oxidative stress. Recent advances in understanding GSNOR regulation show promise for the modulation of oxidative stress in plants.

Molecular functions of nitric oxide and its potential applications in horticultural crops.

Nitric oxide (NO) regulates plant growth, enhances nutrient uptake, and activates disease and stress tolerance mechanisms in most plants, making NO a potential tool for use in improving the yield and quality of horticultural crop species. Although the use of NO in horticulture is still in its infancy, research on NO in model plant species has provided an abundance of valuable information on horticultural crop species. Emerging evidence implies that the bioactivity of NO can occur through many potential mechanisms but occurs mainly through S-nitrosation, the covalent and reversible attachment of NO to cysteine thiol. In this context, NO signaling specifically affects crop development, immunity, and environmental interactions. Moreover, NO can act as a fumigant against a wide range of postharvest diseases and pests. However, for effective use of NO in horticulture, both understanding and exploring the biological significance and potential mechanisms of NO in horticultural crop species are critical. This review provides a picture of our current understanding of how NO is synthesized and transduced in plants, and particular attention is given to the significance of NO in breaking seed dormancy, balancing root growth and development, enhancing nutrient acquisition, mediating stress responses, and guaranteeing food safety for horticultural production.

Root growth inhibition by NH(4)(+) in Arabidopsis is mediated by the root tip and is linked to NH(4)(+) efflux and GMPase activity.

Root growth in higher plants is sensitive to excess ammonium (NH(4)(+)). Our study shows that contact of NH(4)(+) with the primary root tip is both necessary and sufficient to the development of arrested root growth under NH(4)(+) nutrition in Arabidopsis. We show that cell elongation and not cell division is the principal target in the NH(4)(+) inhibition of primary root growth. Mutant and expression analyses using DR5:GUS revealed that the growth inhibition is furthermore independent of auxin and ethylene signalling. NH(4)(+) fluxes along the primary root, measured using the Scanning Ion-selective Electrode Technique, revealed a significant stimulation of NH(4)(+) efflux at the elongation zone following treatment with elevated NH(4)(+), coincident with the inhibition of root elongation. Stimulation of NH(4)(+) efflux and inhibition of cell expansion were significantly more pronounced in the NH(4)(+)-hypersensitive mutant vtc1-1, deficient in the enzyme GDP-mannose pyrophosphorylase (GMPase). We conclude that both restricted transmembrane NH(4)(+) fluxes and proper functioning of GMPase in roots are critical to minimizing the severity of the NH(4)(+) toxicity response in Arabidopsis.